A. Field of Invention
This invention pertains to a transducer for sensing the wheels of a railroad car moving along a track, and more particularly to a transducer capable of sensing said wheel even at very low speeds which transducer includes a self-calibration feature compensating for long term baseline voltage drifts.
B. Description of the Prior Art
Wheel transducers are used frequently along railroad tracks for detecting the wheels of a moving car, frequently in conjunction with safety equipment. For example, railroad crossings are frequently equipped with automatic gates coupled to wheel transducers. The gates close when the wheels of a train are detected by a transducer, and then open after the train passes. Other safety equipment such as bearing and wheel heat sensors are also activated by such transducers upon determining the approach of a train. One such detector is disclosed in U.S. Pat. No. 3,151,827 to Gallagher. One problem with the wheel transducer described in the Gallagher patent has been that the transducer cannot detect with certainty the wheels of a train moving at relatively slow speeds such as below approximately 6 mph. A further problem with prior art transducers has been that a single transducer cannot indicate the direction of movement of a train and hence a pair of spaced apart transducers has been required to determine train direction from the sequence of activation.
Other transducers are known which can detect slow moving trains, however these transducers produce a baseline voltage which tends to drift from a normal level because of temperature changes and other variables affecting the performance of the transducer components. As a result the prior art transducer needed periodic recalibration. The elimination of this drift is particularly important in zero-speed wheel transducers, i.e. transducers with the capability of detecting a train moving very slowly or stopped for an indefinite time period. These types of transducers require a true D.C. coupling and any slow drift in the baseline voltage is indistinguishable from and therefore erroneously interpreted as a slow-moving train.